Pathological Studies of A / Chicken / Tehran / ZMT - 173/99 (H9N2) Influenza Virus in Commercial Broiler Chickens of Iran

Abstract

Avian influenza (AI) outbreaks due to H9N2 subtype of avian influenza virus (AIV) occurred in poultry industry in Iran, in 1998 and caused serious economic losses. The aim of this study was to investigate the pathogenesis, clinical signs, gross and histopathological findings of the chickens experimentally inoculated with A/Chicken/Tehran/ZMT-173/99 (H9N2) influenza virus, isolated from the kidney of the broiler chickens with 40% mortality. Two groups of 30-day-old, forty Mycoplasma gallisepticum positive (Mg+) and forty negative (Mg-) commercial broiler chickens were used. Each group subdivided into 10-membered two experimental and two control subgroups. One experimental subgroup inoculated intravenously (IV) and the other both IV and oculonasally (ON) with 10^7.5 ELD50. Clinically, depression, crouching, huddling, ruffled feathers, coughing, sneezing, and sometimes gasping were observed. Mortality rate was 10% in each Mg+ experimental subgroup. The gross lesions in dead birds included exudative (fibrinous) casts in tracheal bifurcation, pulmonary congestion, thickened air sacs, swollen kidney with urate deposition, enlarged congested and hemorrhagic bursa of fabricious and thymus, petechial hemorrhages in epicardial fat and general congestion in the carcasses. The visceral organs were congested and edematous. Fibrinous tracheal casts in bifurcation were only observed in a chicken inoculated via both IV and ON routs. Histopathologic examination revealed extensive pulmonary hyperemia with infiltration of mononuclear inflammatory cells in lamina propria of bronchi and lungs, mild lymphocytolysis and atrophy of lymphoid follicles of the bursa of fabricious. Severe congestion, depletion of lymphocyte population and focal necrosis were seen in thymus lobules. Severe congestion, urate deposition and nonsuppurative focal interstitial nephritis was the predominant histological lesions. The results of the present study suggest renal lesions and tracheal casts are the principal causes of mortality and simultaneous inoculation play an important role in formation of tracheal casts. Also, immunosuppression due to depletion of lymphoid organs as well as concurrent infections such as M. gallisepticum might increase the pathogenic potential of H9N2 subtype to chicken.